I know exactly know what you mean, when I started running and even today still. There are two different groups that I run with and like you, I am the newest member. There has been more than one time that the "rabbits" ran off, but my pace was faster than the slower people in the group, making me run by myself. I take it as a challenge each week to see how long I can keep the same pace as the seasoned runners, even though they are just "cruising' along, I am getting another step closer to catch them in a race. From what I have found out, the friendships that are build from the groups are not based on times, or years of running. Runners most of the times love talking about running and they will notice if you are improving or having issues.

I run with a coach/running group 3 of my 5 days of running.  We have every level from a girl who just did a olymipic qualifying marathon to over 10:00 a mile pace.  The coach gives us our group to run with and our pace for whatever distance we are running.  It has been great for me, all 4 of my family members run with this group, and each of us keeps getting faster every year and staying healthy.  My times have drop a good bit I can hold 7:20 for a 1/2 when I started I would have a hard time holding 10:00.  And the faster people support the slowest people, I just have a lot of new freinds.  The gorup runs a lot of races together, which helps you a lot.  It works for me and the 30-40 people I run with.